1. Field of the Invention
The invention relates in general to a battery voltage equalization circuit, and in particular, to a battery voltage equalization circuit using a ramp converter.
2. Related Art
It appears that battery packs consisting of several 12 V batteries connected in series to provide a 300-400 V.dc power source, for example, will probably be the most popular battery pack for electric vehicles (EV's) and hybrid electric vehicles (HEV's).
Unfortunately, slight variations exist between individual batteries, and after several charge-discharge cycles, the battery voltages between the individual batteries begin to vary. If a battery voltage becomes too high during charging, the battery can no longer store all of the input charge, and "gassing" and overheating may occur. Gassing represents a loss of an electrolyte, and the overheating is detrimental to the electrodes. If a battery voltage becomes too low during discharge, the acidic concentration of the electrolyte may be diminished to a very low level. This can also damage the electrodes and shorten battery life.
To avoid these problems, a battery management system may be used to monitor the voltages of the individual batteries and take corrective action when any of these voltages reach their high or low limit. Corrective action consists of stopping the discharge at the lower limit and decreasing or stopping the charge at the higher limit. However, this also means a single low voltage battery can force a premature shutdown and greatly diminish the range of the vehicle. Likewise, a single high voltage can force a reduction in charging current and greatly increase the required charging time. Therefore, it is important to incorporate some equalization process for keeping all of the battery voltages close to the same value.
There are a wide variety of equalization strategies and an even wider variety of implementations, some of which will be described below. The high current portion of the charge period is called the "bulk charge" since this is where the battery receives most of its energy. The remaining low current portion (perhaps 1 Ampere, or so) is called the "trickle charge."
The most common equalization method is to simply allow the batteries to trickle charge for a few hours after the bulk charge is complete. This low current increases the charge in the undercharged, low voltage batteries without causing excessive heating or gassing in the fully charged, high voltage batteries. This equalizing method is effective, but it requires an excessive charge time.
Electro-mechanical equalizers can be used to monitor all of the battery voltages and selectively connect the output of a transformer coupled DC-DC Converter to provide additional charge to the lowest voltage battery(s). These equalizers are so named because a set of electro-mechanical relays seems to be the most practical means of connecting the converter to the individual batteries. These units also are effective, but they are relatively complex and expensive.
Most electronic equalizers consist of a transformer coupled DC-DC converter with several secondary windings and rectifiers. Usually there is a separate secondary winding and rectifier for each battery, but other combinations are possible. The converter is operated in the constant current mode, and the circuit is arranged so that each battery is effectively connected in parallel with the converter output. This means the lower voltage batteries will "hog" the output current and thus receive an extra charge to increase their voltages. These equalizers are much simpler and cheaper than the electro-mechanical types, and tests indicate they can provide very effective equalization.
Although electronic equalizers offer certain advantages over the electro-mechanical variety, several of the previous units have fairly high switching losses, and the large number of secondary windings complicates the transformer construction.